Osteoporosis affects approximately 25% of women over the age of 60, with an estimated 540,000 women experiencing non-traumatic vertebral fractures every year. Currently, there is no effective treatment for vertebral crush fractures, highlighting the importance of preventing this condition. Moderate exercise is frequently mentioned as a means of maintaining or even increasing bone mineral density (BMD) and possibly preventing the development of osteoporotic fractures. Unfortunately, this recommendation is based on a limited number of studies that have measured the effects of various exercise training program on BMD in post menopausal women (PMW). Furthermore, the minimal amount of exercise needed to increase BMD is unknown. Before individuals at risk for developing osteoporosis can be counseled on means to reduce their risk, quantified information on the effectiveness of various prophylactic measures is needed. The current study proposes to examine two questions: 1) what is the dose-response relationship between aerobic exercise and BMD in naturally PMW and, 2) what are the effects of aerobic exercise on exercise capacity in naturally PMW? Sedentary, naturally PMW will be recruited and screened for the presence of conditions that might influence calcium metabolism or invalidate the bone densitometry test. Sixty subjects free from these conditions will then be randomly assigned to either a non-exercising control group or to one of two exercising groups (20 subjects per group) and given 750 mg/day of elemental calcium. The exercise groups will train three times per week for one year at 70-85% of their maximal heart rate on motorized treadmills for either 30 or 45 minutes per session. BMD of the lumbar vertebrae (L1-L4) and radius will be measured with dual photon absorptiometry and single photon absorptiometry, respectively. BMD will be evaluated at baseline and following 6 and 12 months of training. Maximal oxygen uptake will be measured at baseline and following 3, 6 and 12 months of training. This study will expand upon the existing literature by evaluating a homogenous sample of naturally PMW, free of conditions that might influence calcium metabolism or invalidate DPA bone densitometry and provide information on two questions: 1) what is the dose response relationship between aerobic exercise and BMD in naturally PMW and, 2) what are the effects of long-term aerobic exercise training on exercise tolerance in sedentary, naturally PMW?